Update Postfix Integration doc
Update Postfix Integration doc

Moved password to secured sasl_passwd file, together with
necessary steps to compile such file.
Also updated service restart command to systemd.
Whatthefoxsays/docs,sendgrid/docs,Aonepathan/docs,katieporter/docs,hardikjain29/docs,katieporter/docs,ksigler7/docs,katieporter/docs,sendgrid/docs,Aonepathan/docs,hardikjain29/docs,hardikjain29/docs,Aonepathan/docs,ksigler7/docs,katieporter/docs,hardikjain29/docs,Aonepathan/docs,sendgrid/docs,ksigler7/docs,katieporter/docs,Whatthefoxsays/docs,Whatthefoxsays/docs,Aonepathan/docs,hardikjain29/docs